Thin plate having excellent corrosion resistance, conductivity and formability, and method for manufacturing same

ABSTRACT

[Problem] To prepare a thin plate having excellent corrosion resistance, conductivity, and formability at low cost. 
     [Solution] A thin plate is prepared by an ultraquenching transition control injector with a mixture of a metal powder having corrosion resistance to form a matrix and a powder having conductivity, as a raw material. An obtained thin plate has a conductive material component that exists, without dissolving, in a metal matrix exhibiting corrosion resistance by passivation, thereby having aforementioned characteristics.

TECHNICAL FIELD

The present invention relates to a thin plate having excellent corrosionresistance, conductivity and formability as an industrial material, andthe manufacturing method therefor.

BACKGROUND ART

A polymer electrolyte fuel cell (PEFC) extracts energy as electric powerfrom the reaction between hydrogen and oxygen to generate water, whichis expected to be useful for future society as a clean power source withno CO₂ emission. The use of PEFCs, such as for automobiles, householduse fuel cells, and mobile phones, is well known. A component called aseparator is used inside the PEFCs. This is mainly a component thatforms a flow path for hydrogen and oxygen and allows electricity to flowbetween cells.

For the material of PEFC separators, two kinds of material, carbon andmetal, are generally used. On the basis of the reasoning that carbon ispoor in workability and thick, thus increasing size, metallic separatorsare anticipated in automotive PEFCs. The development thereof has beenunderway not only at manufacturers but also at research institutionssuch as universities.

In Non Patent Literature 1, formability, corrosion resistance, contactresistance, and power generation characteristics at a supercooled liquidtemperature range are reported for the separator of a metallic glassmaterial.

Patent Literature 1 discloses a manufacturing method in which stainlesssteel is applied as a base, and in order to make the stainless steelconductive as well, a passive layer is passed through with a deposition,thereby increasing the conductivity between the stainless interior andits surface. A passive layer has high electric resistance, so thatcontact resistance increases (conductivity deteriorates) when thesurface of a material is covered therewith.

Also in Patent Literatures 2 and 3, the material, in which a passivelayer is formed on a surface thereof to improve corrosion resistance, isselected, and similarly as described above, special treatment such asplating is performed on the surface in order to improve conductivity.

In Patent Literature 4, the manufacturing apparatus for preparing anamorphous thin plate and the method therefor are disclosed, allowing fora thin plate in a size required for PEFC separators. FIG. 1 shows thestructure of an injection gun as a primary device. With this injectiongun, a film is formed on a substrate surface while flying powderparticles are quenched, finally followed by the release of the film fromthe substrate so as to obtain an amorphous thin plate.

CITATION LIST Non Patent Literature

-   [NPL 1] Masanori Yokoyama, Shinichi Yamamura, Hisamichi Kimura,    Akihisa Inoue; “Hot Press Workability of Ni-based Metallic Glass in    Supercooled Liquid State and Prototype of Fuel Cell Separators,”    Powder and Powder Metallurgy, 54 (2007), 773-777

Patent Literature

-   [PTL 1] Japanese Unexamined Patent Application Publication No.    2003-193206-   [PTL 2] Japanese Unexamined Patent Application Publication No.    2006-210320-   [PTL 3] Japanese Unexamined Patent Application Publication No.    H10-228914-   [PTL 4] Japanese Patent Publication No. 4579317

SUMMARY OF INVENTION Technical Problem

The practical use of metallic separators for PEFCs has been considereddifficult because of problems in obtaining a material satisfying allcorrosion resistance, conductivity, formability, and cost. In Non PatentLiterature 1 and Patent Literature 1, it is said that non-conductiveoxide grows on a material surface when applying current, so that powergeneration characteristics degrade. In Patent Literatures 2 and 3, thereseems to be a cost issue for practical uses because special treatment isrequired for a surface and an expensive material is used. The presentinvention proposes a metallic separator for PEFCs that satisfies allcorrosion resistance, conductivity and formability without requiring acomplex procedure, or a thin plate applicable as a material for theseparator and the like, and a manufacturing method therefor.

Solution to Problem

In the present invention, the ultraquenching transition control injectoras shown in FIG. 1 or FIG. 2 is used so as to prepare a metallic thinplate. These injectors are capable of forming, by using a powder rawmaterial, a quenched coating of a melted powder material, on a substratesurface, and further preparing a quenched thin plate obtained byreleasing the coating from the substrate. When a powder material havinga composition which can easily become amorphous is used, the injector iscapable of obtaining an amorphous coating and thin plate. Specifically,the powder material that was completely melted in flame during flying isquenched with refrigerant such as nitrogen gas and mist before reachingthe substrate. As a result, the coating formed on the substrate surfacewill be amorphous. The difference between the ultraquenching transitioncontrol injectors in FIG. 1 and FIG. 2 is the width of coatings formedat a time. In FIG. 1, the width is 15 mm, and in FIG. 2, the width is300 mm. With either ultraquenching transition control injector, coatingsand thin plates of the same quality may be obtained. However, inconsidering production efficiency per ultraquenching transition controlinjector, the ultraquenching transition control injector in FIG. 2 ismore suitable, so that in the present invention, mainly this injector isused to prepare thin plates.

In the present invention, for the powder material supplied to theabove-mentioned ultraquenching transition control injector, conductivepowder is mixed into the composition that tends to become amorphous, orthe like.

Normally in the process of solidifying the metal having a compositionthat becomes amorphous, when there is a certain substance resulting in acore of crystallization, such as the conductive powder that is mixedthis time, the metal is easily crystallized, instead of being amorphous,and then solidified. However, the use of the ultraquenching transitioncontrol injector as in FIG. 1 and FIG. 2 can prevent crystallization.This is because the metal powder having a composition that becomesamorphous and the conductive powder are not in a mixed state duringflying, but these powders fly discretely and (the metal powder having acomposition to be amorphous) solidify. That is, by using theultraquenching transition control injector as in FIG. 1 or FIG. 2 withthe powder material in which the metal powder having the compositionthat can become easily amorphous and the conductive powder are mixed, itis possible to prepare a thin plate having the conductive material mixedin a metal matrix which is amorphous or includes an amorphous structure.

For this, in order to actually confirm this beforehand, Ni₆₅Cr₁₅P₁₆B₄ asthe metal powder having a composition that becomes amorphous and C, B₄Cas the conductive powder were selected, and an amorphous thin plate wasprepared by the above-described method with the use of theultraquenching transition control injector in FIG. 1. When an amorphousrate was calculated from the thermal energy of the thin plate measuredby DSC (with the thermal energy of the amorphous Ni₆₅Cr₁₅P₁₆B₄ ribbon as100%), the thin plates mixed with either C or B₄C conductive powder hadthe amorphous rate of 89 to 95%. This result is the same as theNi₆₅Cr₁₅P₁₆B₄ thin plate without mixed conductive powder. It wasconfirmed that the amorphization of Ni₆₅Cr₁₅P₁₆B₄ was not affected bymixed conductive powder. It is noted that DSC stands for DifferentialScanning Calorimetry, which measures a difference in amount of heatbetween a measurement sample and a reference material. With thismeasurement, it is possible to obtain a thermal energy value when ameasurement sample is crystallized from the amorphous state.

For a metal material having corrosion resistance as a base material(matrix), the use of amorphous and crystal-structure metal such asstainless steel can be considered. Metallic glass, though amorphous itis, has a temperature range in which to turn into a supercooled liquid,so that if the metallic glass is formed in this temperature range, it ispossible to perform processing at excellent dimensional accuracy withoutgenerating cracks. Also, in the stainless steel, in a case where aplate-shaped product is produced with a general manufacturing method,when boron (B) or the like is added in large quantity to form aconductive deposition, boron (B) or the like dissolves in a matrix in alarge amount to deteriorate workability (Solution Hardening). However,when the above-described manufacturing method using the ultraquenchingtransition control injector in FIG. 1 or FIG. 2 is used, the componentof the conductive material powder to be mixed does not dissolve in astainless matrix and exists solely in a thin plate, so that workabilitydeterioration is not expected to be generated. FIG. 4 shows across-sectional picture of a thin plate that was actually prepared. Thisis the cross-sectional picture of a thin plate that was formed from thepowder material, which was the mixture of SUS316L powder and 2.5 wt % ofB₄C, with the ultraquenching transition control injector in FIG. 2. Adark gray area in a dotted circle is B₄C. Furthermore, FIG. 5 shows aresult of an EDX analysis of SUS316L matrix points in thiscross-sectional picture. It was actually confirmed that B did notdissolve in the stainless matrix as described above because the peak ofB was not detected herein. EDX stands for Energy Dispersive X-raySpectrometry. This is an element analysis method, which detects X-raysgenerated when a sample is irradiated with electron beams, with anenergy dispersive detector, and examines substances that make up thesample and its concentrations on the basis of the energy and intensityof the X-rays. In FIG. 5, the horizontal axis corresponds to energy; andthe vertical axis corresponds to intensity.

The metal materials to be examined in the present invention form apassive layer to exhibit corrosion resistance, so that the contactresistance is considered large unless treated. Therefore, in order toreduce the contact resistance, it is required to mix a conductivematerial powder allowing electricity to flow in the passive layer.

For the conductive powder material, the use of a non-metallic C-basedpowder was considered. The reason for this is that most of thenon-metallic conductive powders remain stable at ph=3 and 80° C. as thedriving environment of PEFC, and these powders are inexpensive; however,as long as cost and the effect of characteristics match, variouscomponents may be used for the conductive powder. That is because theultraquenching transition control injector for use in the presentinvention melts a metal powder material at about 2000° C. with reducingflame with a reduced supply of oxygen, so that when the melting point ofconductive powder is higher than that temperature, the conductive powdercan remain in a thin plate without melting. FIG. 4 shows a specificexample of a thin plate that was actually prepared by the ultraquenchingtransition control injector. It is possible to find the remaining B₄C(in a dotted circle) in the thin plate, and this B₄C is also one of thecomponents to be used as the conductive powder.

Advantageous Effects of Invention

The present invention enables the production of metallic separators forPEFCs having excellent corrosion resistance, conductivity, andformability at low cost. As for the thickness of thin plates, in a caseof using steel coils, an increase in rolling costs may be considered asthe plates become thinner. However, in the manufacturing method of thepresent invention, a reduction in plate thickness can be easily adjustedby decreasing the feed rates of the material powder, decreasing therelative speeds between a substrate and an injection gun, etc.

The thin plate of the present invention has high corrosion resistance ata matrix portion and has excellent conductivity because of having aconductive material. The plate is also advantageous in terms offormability and manufacturing costs, so that it is highly suitable as ametallic separator for PEFCs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view showing the usage of an ultraquenching transitioncontrol injector.

FIG. 2A is a side view and FIG. 2B is a bottom view each showing a largeultraquenching transition control injector.

FIG. 3 is a side view showing the overall production line of thinplates.

FIG. 4 is microscopic structure photography showing the cross section ofa SUS316L thin plate mixed with B₄C at 2.5 wt %.

FIG. 5 is a graph showing the EDX result of a SUS316L matrix of FIG. 4.

FIG. 6 is a side view showing upstream and downstream mist angles of theultraquenching transition control injector of FIG. 2.

FIG. 7 is microscopic structure photography showing the cross section ofa Ni₆₅Cr₁₅P₁₆B₄ thin plate mixed with C at 0.3 wt %.

FIG. 8 is a diagram illustrating a circuit 1 of measurement.

FIG. 9 is a diagram illustrating a circuit 2 of measurement.

FIG. 10 is a graph showing the result of contact resistance measurement.

DESCRIPTION OF EMBODIMENTS

1. Preparation of Thin Plate and Test Specimen (Corrosion Resistance,Contact Resistance)

For a metal material for use in an injection gun (ultraquenchingtransition control injector), a gas atomized powder having thecomposition of Ni₆₅Cr₁₅P₁₆B₄ (at %) and being classified as +38/−63 μmin diameter was used. This is a composition that solidifies as metallicglass when quenched, and this composition was selected also in thepresent invention in order to achieve the formation in a supercooledliquid range.

For a conductive powder to be mixed in the metal material mentionedabove, artificial graphite (AGB-5 from Ito Graphite Co., Ltd.) havingthe average particle size of 5 μm was used (hereinafter, referred to ascarbon). This powder is obtained by pulverizing artificial graphiteelectrodes, and is available at low cost.

Ni₆₅Cr₁₅P₁₆B₄ and 0.3 wt % of carbon powder were mixed and stirred toobtain a material for injection. After mixing, water was removed bykeeping the material warm in a drying oven under the condition of 80° C.for two hours. This is performed for a purpose of achieving stablepowder supply without clogging or the like inside a supply path duringthe injection of the powder material.

For the injection gun, the ultraquenching transition control injectorshown in FIG. 2 was used. Mixed gas of oxygen and propane is used forfuel, and combustion flame is ejected from flame vents 5 outside ofpowder ejection ports 6, which are arranged at equal intervals in thewidthwise direction. The powder material ejected from the powderejection ports 6 is completely melted once in the combustion flame. Thematerial is deposited on the surface of a substrate while beingquenched, right after being melted, with a refrigerant mist ejected froma mist ejection port 3 arranged outside the powder ejection ports 6,thereby forming a film. This injection gun injects the materialuniformly in the widthwise direction, and thus, it is possible toprepare a thin plate having a uniform thickness in the widthwisedirection.

The above-described ultraquenching transition control injector wasinstalled in the thin plate production line shown in FIG. 3. A pickledsteel coil of 2 mm in thickness×300 mm in width is set between a payoffreel 7 and a coil winder 13, and the coil is moved toward the coilwinder 13. First, the coil is heated with propane flame by a preheater8. Then, after the coil shape was corrected by a leveler 9, the coil washeated up to the target temperature of 250° C. by a thin plate substrateheating and heat-equalizing device 10.

On the surface of the coil that was heated up to the target temperatureof 250° C., a film was formed with the mixed powder by an ultraquenchingtransition control injector 11. Immediately after the film formation,10% reduction was applied thereon with a rolling mill 12. Before beingwound on the coil winder 13, the film was released from the coil, thusobtaining a thin plate 14. At this time, the film temperature is 220 to280° C. right after the reduction applied with the rolling mill 12. Itis noted that in a series of operations, a coil speed was constant at5.7 m/min. The above-described condition for manufacturing thin platesis shown in Table 1. An upstream mist angle and a downstream mist anglein Table 1 show a positional relationship, relative to the coil movementdirection, of a mist ejection nozzle 2 and an inclination from thedirection at right angles to the plane of the coil. The conditions areillustrated in FIG. 6.

TABLE 1 Feed Rate of Alloy Powder (g/s) 30 Flow Rate of Propane Gas(m³/h) 34 Oxygen Flow Rate (m³/h) 120 Rectified Nitrogen Flow Rate(m³/h) 400 Injection Distance to Coil (mm) 600 Upstream Mist Angle (°) 9Upstream Mist Flow Rate (liter/min) 4 Downstream Mist Angle (°) 9Downstream Mist Flow Rate (liter/min) 4 Coil Surface Temperature Before250 Injection Rolling Speed (m/min) 5.7

The thin plate obtained thereby had a size of 300 μm in thickness×300 mmin width. The thin plate was confirmed by DSC to have 85% amorphous ratein comparison with an amorphous ribbon material which has 100% amorphousrate. FIG. 7 is a cross-sectional picture of the thin plate obtainedthereby. It is observed that C (in a dotted circle) remains in theNi₆₅Cr₁₅P₁₆B₄ matrix.

Further, in order to confirm a difference in contact resistance betweenthe cases with or without the conductive powder, an amorphous thin platewas also prepared from Ni₆₅Cr₁₅P₁₆B₄ powder having no mixed carbonpowder, in the same procedures as described above. Finally, thefollowing two types of thin plates were prepared.

TABLE 2 Mixing Rate of Composition of Metal Conductive Conductive PowderSample No. Matrix (at %) Powder (wt %) 1 Ni₆₅Cr₁₅P₁₆B₄ None — 2Ni₆₅Cr₁₅P₁₆B₄ C 0.3

2. Contact Resistance Test

The prepared thin plates were cut out in the size of 35-mm square with amicro cutter. The amorphous thin plates were treated by a router to havea flat and smooth surface on the side opposite to the coil (having had asurface roughness of about Ra 10 μm since they remained as they wereafter injection).

In order to passivate the material surface, the plates were immersed fortwo hours in sulfuric acid of ph=3 at 80° C. and then experimented on.

A constant current of 1 A was applied to a circuit shown in FIG. 8 tomeasure a potential difference between gold layers (Au-1-Au-2), and aresistance value was calculated on the basis of Ohm's law. Thisresistance value, being a contact resistance of Au-Carbon (C) paperexisting at two locations in the circuit, was divided by 2 to give Rc(contact resistance of Au—C paper). To determine Rc per 1 kgf/cm²,contact pressure was changed from 1 to 7 kgf/cm².

Then, a constant current of 1 A was similarly applied to a circuit shownin FIG. 9 to measure a potential difference between Au-1 and the testspecimen, and a resistance value Ra was calculated on the basis of Ohm'slaw. Similar to the above description, to determine Ra per 1 kgf/cm²,contact pressure was changed from 1 to 7 kgf/cm². Finally, according tothe formula below, a contact resistance value Rs between the testspecimen and the C paper was calculated, and conductivity was evaluatedon the basis of this value.Rs=Ra−Rc

FIG. 10 shows the measurement result of contact resistance. Theamorphous material of Ni₆₅Cr₁₅P₁₆B₄ exhibits corrosion resistance bypassivation on its surface. On the surface of both test specimens, apassive layer is formed during the sulfuric acid immersing processbefore the test. A reason why the contact resistance value of the Cadmixture is low at any contact pressure may be that C, a conductor, ispresent also in the passive layer. Accordingly, it is possible to saythat the contact resistance that increases with passivation may bereduced by preparing a metallic thin plate with a powder material mixedwith C by using an ultraquenching transition control injector.

3. Corrosion Resistance Test

The prepared No. 2 thin plate (with mixed C) was cut out in the size of20-mm square with a micro cutter, and was then experimented. As animmersion solution, sulfuric acid of ph=3 (80° C.) was prepared, andimmersion was performed for 24 hours. The weight of the test specimenwas measured before and after immersion, and corrosion rate (μm/year)was calculated from weight changes and specific gravity.

The result was 3 μm/year, confirming that it had sufficient corrosionresistance as a separator for PEFCs.

As described above, it was confirmed that a thin plate with a mixedconductive powder of the present invention can satisfy conductivity andcorrosion resistance necessary for a separator for PEFCs.

REFERENCE SIGNS LIST

-   -   1 Powder Supply Pipe    -   2 Mist Ejection Nozzle    -   3 Mist Ejection Port    -   4 Inert Gas Ejection Port    -   5 Flame Vent    -   6 Powder Ejection Port    -   7 Payoff Reel    -   8 Preheater    -   9 Leveler    -   10 Thin plate substrate heating and heat-equalizing device    -   11 Ultraquenching Transition Control Injector    -   12 Rolling Mill    -   13 Coil Winder    -   14 Released Thin Plate

The invention claimed is:
 1. A method for manufacturing a thin compositeplate, the thin composite plate comprising a conductive materialcomponent that exists, without dissolving, in an amorphous metal matrixexhibiting corrosion resistance by passivation, the method comprising:providing an injection gun having a plurality of powder ejection portsarranged at equal intervals in a widthwise direction, a plurality offlame vents arranged outside of the powder ejection ports, and aplurality of mist ejection ports arranged further outside of the flamevents for cooling, injecting metal used for the metal matrix and theconductive material from the powder ejecting ports of the injection gunwith flame ejected from the flame vents toward a substrate so that themetal is melted, and then cooling the metal and the conductive materialwith refrigerant mists ejected from the mist ejection ports before theflame reaches the substrate so as to provide a composite plate with acoating laminated on the substrate.
 2. The method for manufacturing athin plate from the thin composite plate formed according to claim 1,which comprises releasing the coating from the composite plate toprovide the thin plate.
 3. The method for manufacturing the thincomposite plate, according to claim 1, wherein metal having acomposition that becomes amorphous by quenching is used as the metal forthe metal matrix, to make the metal matrix in the coating include anamorphous structure.
 4. The method for manufacturing the thin plate,according to claim 2, wherein metal having a composition that becomesamorphous by quenching is used as the metal for the metal matrix, tomake the metal matrix in the coating and the thin plate include anamorphous structure.
 5. The method of claim 2, wherein the conductivematerial component is C or B₄C, and the metal used to form the metalmatrix is Ni₆₅Cr₁₅P₁₆B₄.
 6. A metallic separator for a polyelectrolytefuel cell (PEFC) comprising a thin plate made by the method of claim 5.7. The method for manufacturing the thin composite plate, according toclaim 1, which comprises subjecting the composite plate to reductionwith a rolling mill.
 8. The method for manufacturing a thin plate fromthe thin composite plate according to claim 7, which comprises pressingthe composite plate by a rolling mill to release the coating therefromto provide the thin plate.
 9. The method for manufacturing a thincomposite plate according to claim 7, wherein the rolling mill has anupper roll and a lower roll with a different diameter from each other,and one of the rolls which works in contact with the coating has asmaller diameter than the other roll which works in contact with thesubstrate.
 10. The method for manufacturing the thin plate from the thincomposite plate according to claim 8, wherein a coil as the substrate isset between a payoff reel and a coil winder, and is moved toward thecoil winder, and while the coil is moved, heating of the coil, saidinjections, said reduction with the rolling mill, and said release ofthe coating are performed in this order.
 11. The method formanufacturing a thin plate manufactured from the thin composite plateaccording to claim 1, wherein the metal matrix shows an amorphous ratioof 85% or more when measured by a differential scanning calorimetry(DSC).
 12. A metallic separator for a polyelectrolyte fuel cell (PEFC)comprising a thin plate made by the method of claim 8.